1. Field of the Invention
The present invention relates to a heat-sensitive recording material and more particularly to a heat-sensitive recording material excellent in lightfastness.
2. Description of the Related Art
In recent years, heat-sensitive recording has made remarkable progress because the recording apparatus for heat-sensitive recording are simple, highly reliable, and maintenance-free. Examples of widely known heat-sensitive recording materials are a heat-sensitive recording material utilizing the reaction between an electron-donating colorless dye and an electron-accepting compound, and a heat-sensitive recording material utilizing the reaction between a diazonium salt compound and a coupler.
However, the heat-sensitive recording material is associated with a problem that, when it is exposed to sunlight for a long time or when it is posted on the wall in an office or the like for a long period of time, its background becomes colored and the image portions may discolor or fade.
Japanese Patent Application Laid-Open (JP-A) No.9-1928 describes a heat-sensitive recording material which uses a precursor of an ultraviolet light absorber in order to prevent the coloration of the background and the discoloration or fading of the image portions and exhibits excellent lightfastness and fixing performance. This precursor of an ultraviolet light absorber functions as an ultraviolet light absorber after it reacts due to the effect of light or heat when irradiated with light whose wavelength is in a region necessary for fixing by irradiation with light in a photo-fixing type heat-sensitive recording layer. For this reason, a large proportion of the light within an ultraviolet region is absorbed by the ultraviolet light absorber and the ultraviolet light transmittance of the heat-sensitive recording material is lowered. Therefore, the lightfastness of the heat-sensitive recording material is enhanced.
However, if the precursor of an ultraviolet light absorber was used alone, the problem encountered was that the efficiency of the ultraviolet light absorber formation was low when the ultraviolet light absorber was formed by irradiation with light.
The present invention provides a heat-sensitive recording material which is excellent in lightfastness and solves the problem that the efficiency of the ultraviolet light absorber formation is low and thus overcomes the above-mentioned problems.
That is, the first aspect of the present invention is a heat-sensitive recording material containing an ultraviolet light absorber precursor which forms an ultraviolet light absorber by being irradiated with light and a hydrogen donor. Preferably, the ultraviolet light absorber precursor is at least one kind selected from the compounds represented by the following general formulae (1) to (4). 
In the general formulae (1) to (4), m represents 1 or 2. A represents xe2x80x94SO2xe2x80x94R, xe2x80x94COxe2x80x94R, xe2x80x94CO2xe2x80x94R, xe2x80x94CONHxe2x80x94R, xe2x80x94POR1R2, xe2x80x94CH2R3, or SiR4R5R6 in the general formula (1) where m is 1 and in the general formulae (2) to (4). A represents xe2x80x94SO2R7SO2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COCOxe2x80x94, xe2x80x94COR7COxe2x80x94, xe2x80x94SO2xe2x80x94, or SOxe2x80x94 in the general formula (1) where m is 2. X represents a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, or a halogen atom in the general formulae (1), (3), and (4). X represents an alkylene group, xe2x80x94OR7Oxe2x80x94, or OCOR7CO2xe2x80x94 in the general formula (2). W represents a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, or a halogen atom in the general formulae (1), (2), and (4). W represents xe2x80x94OR7Oxe2x80x94 or OCOR7CO2xe2x80x94 in the general formula (3). Y represents a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, or a halogen atom in the general formulae (1), (2), and (3). Y represents xe2x80x94OR7Oxe2x80x94, xe2x80x94OCOR7CO2xe2x80x94, xe2x80x94CH2CH2CO2R7OCOCH2CH2xe2x80x94, xe2x80x94CH2CH2OCOR7CO2CH2CH2xe2x80x94, or CH2CH2CON(R8) R7N(R8)COCH2CH2xe2x80x94 in the general formula (4). Z represents a hydrogen atom, a halogen atom, an alkyl group, or an alkoxy group. In the groups listed above, R represents an alkyl group or an aryl group. R1 and R2 each represents an alkoxy group, an aryloxy group, an alkyl group, or an aryl group. R3 represents a phenyl group bearing at least one nitro or methoxy group as a substituent. R4, R5, and R6 each represents an alkyl group or an aryl group. R7 represents an alkylene group or an arylene group. R8 represents a hydrogen atom or an alkyl group.
It is preferable that the hydrogen donor has at least one group selected from an aromatic OH group, an aliphatic OH group, a heterocyclic OH group, an aromatic SH group, an aliphatic SH group, a heterocyclic SH group, an aromatic NH group, an aliphatic NH group, and a heterocyclic NH group.
Another aspect of the present invention is a heat-sensitive recording material comprising a support, a photo-fixing type heat-sensitive recording layer, an interlayer, and a protective layer, each provided on the support, wherein at least one of these layers contains an ultraviolet light absorber precursor and a hydrogen donor.
Still another aspect of the present invention is a multicolor heat-sensitive recording material which comprises a support and a plurality of photo-fixing type heat-sensitive recording layers provided on the support and contains an ultraviolet light absorber precursor and a hydrogen donor, wherein the photo-fixing type heat-sensitive layers develop the three primary colors for subtractive color mixing, i.e., yellow, magenta, and cyan, respectively, so that full-color recording can be made.
 less than Heat-Sensitive Recording Material greater than 
The heat-sensitive recording material of the present invention contains at least one compound selected from the compounds represented by the following formulae (1) to (4) and a hydrogen donor. 
The compounds represented by the general formulae (1) to (4) are so-called precursors of ultraviolet light absorbers and form ultraviolet light absorbers by being irradiated with light. The compounds represented by the general formulae (1) to (4) are hereinafter referred to as xe2x80x9cultraviolet light absorber precursorsxe2x80x9d upon occasion. The heat-sensitive recording material of the present invention contains a hydrogen donor in addition to the ultraviolet light absorber precursor. For this reason, the supply source, which supplies hydrogen to the ultraviolet light absorber precursor is increased and the efficiency of the formation of the ultraviolet light absorber is enhanced. The term xe2x80x9chydrogen donorxe2x80x9d as used in the present invention means a compound which donates hydrogen to the ultraviolet light absorber precursor.
(Hydrogen Donor)
Examples of the hydrogen donor include hydroquinone compounds, hydrazide compounds, hydroxy compounds, phenidone compounds, catechol compounds, resorcinol compounds, hydroxyhydroquinone compounds, pyrrologlycinol compounds, phenol compounds, phenylhydrazide compounds, gallic acid compounds, ascorbic acid compounds, and ethylene glycol compounds. These are described in JP-A Nos.3-191341, 3-25434, 1-252953, 2-302753, 1-129247, 1-227145, 1-243048, 2-262649, etc.
Among the hydrogen donors listed above, preferable is a hydrogen donor (an OH group-bearing hydrogen donor) having at least one group selected from an aromatic OH group, an aliphatic OH group, and a heterocyclic OH group; a hydrogen donor (an SH group-bearing hydrogen donor) having at least one group selected from an aromatic SH group, an aliphatic SH group, and a heterocyclic SH group; or a hydrogen donor (an NH group-bearing hydrogen donor) having at least one group selected from an aromatic NH group, an aliphatic NH group, and a heterocyclic NH group.
Examples of the OH group-bearing hydrogen donor include compounds represented by xe2x80x9cheterocycle-OHxe2x80x9d, xe2x80x9cR10xe2x80x94COOHxe2x80x9d, etc., wherein R10 represents an aliphatic group, an aromatic group, or a heterocyclic group. Examples of R10 include octyl, dodecyl, octadecyl, 2-ethylhexyl, oleyl, phenyl, naphthyl, tolyl, cyclohexyl, and pyridyl groups. Examples of the OH group-bearing hydrogen donor include hydroquinone and derivatives thereof, phenol and derivatives thereof, hindered phenol and derivatives thereof, tocopherol and derivatives thereof, and ascorbic and derivatives thereof. The OH group-bearing hydrogen donor may have two or more OH groups. Preferred specific examples of the OH group-bearing hydrogen donor are given below. 
Examples of the SH group-bearing hydrogen donor include mercapto compounds represented by xe2x80x9cheterocycle-SHxe2x80x9d and compound represented by xe2x80x9cR11xe2x80x94COSHxe2x80x9d, etc., wherein R11 represents an aliphatic group, an aromatic group, or a heterocyclic group. Examples of R11 include octyl, dodecyl, octadecyl, 2-ethylhexyl, oleyl, phenyl, naphthyl, tolyl, cyclohexyl, and pyridyl groups. Examples of the SH group-bearing hydrogen donor include 2-mercaptobenzimidazole, 2-mercaptobenzoxazole, 2-mercaptobenzothiazole, etc. The SH group-bearing hydrogen donor may have two or more SH groups.
Examples of the NH group-bearing hydrogen donor include compounds represented by xe2x80x9cheterocycle-NHxe2x80x94R12xe2x80x9d, R13xe2x80x94CONHxe2x80x94R12xe2x80x9d, etc., wherein R12 represents a hydrogen atom, an aliphatic group, an aromatic group, or a heterocyclic group and R13 represents an aliphatic group, an aromatic group, or a heterocyclic group. Examples of R12 include a hydrogen atom, and octyl, dodecyl, octadecyl, 2-ethylhexyl, oleyl, phenyl, naphthyl, tolyl, cyclohexyl, and pyridyl groups. Examples of R13 include octyl, dodecyl, octadecyl, 2-ethylhexyl, oleyl, phenyl, naphthyl, tolyl, cyclohexyl, and pyridyl groups. Examples of the NH group-bearing hydrogen donor include primary amines and derivatives thereof, secondary amines and derivatives thereof, and compounds having a urea linkage. The NH group-bearing hydrogen donor may have two or more NH groups. Preferred specific examples of the NH group-bearing hydrogen donor are given below. 
Besides the hydrogen donors listed above, it is possible to use hydrogen donors such as an ester (e.g., triethanolamine acetate) prepared by esterification of an amine having a hydroxyalkyl group as a substituent, an ether having an oxyalkylene group (e.g., 2-benzyloxynaphthalene and 1,2-diphenoxyethane), etc.
Preferably, the heat-sensitive recording material of the present invention comprises a support and a photo-fixing type heat-sensitive recording layer and, if necessary, an interlayer and a protective layer may also be provided on the support. The hydrogen donor may be incorporated in any of these layers, with the proviso that the hydrogen donor is incorporated in the layer containing a compound represented by any of the general formulae (1) to (4). In some case, for example, the heat-sensitive recording material may have an additional layer such as a light transmittance controlling layer which contains the hydrogen donor and the ultraviolet light absorber precursor.
The proportion of the hydrogen donor is preferably 1 to 1000% by weight, more preferably 5 to 300% by weight, and most preferably 10 to 100% by weight relative to the compound represented by any of the general formulae (1) to (4). If the proportion is within the range of 1 to 1000% by weight, the compounds represented by the general formulae (1) to (4) can be converted efficiently into ultraviolet light absorbers.
(Compounds Represented by the General Formulae (1) to (4))
In the present invention, the compounds represented by the general formulae (1) to (4) function as precursors of ultraviolet light absorbers. These compounds do not function as ultraviolet light absorbers before the irradiation with light in the wavelength region necessary for fixing. For this reason, the light transmittance of the photo-fixing type heat-sensitive recording layer containing the precursor of ultraviolet light absorber is high. The precursor of ultraviolet light absorber sufficiently transmits the light in the wavelength region necessary for fixing and does not hinder the fixing of the photo-fixing type heat-sensitive recording layer. In addition, the visible light transmittance in the photo-fixing type heat-sensitive recording layer is also high.
After the compounds represented by the general formulae (1) to (4) are irradiated with light in the wavelength region necessary for fixing by light in the photo-fixing type heat-sensitive recording layer and react due to the effect of light or heat, the compounds function as ultraviolet light absorbers. For this reason, the light in an ultraviolet region is mostly absorbed by the ultraviolet light absorber. Accordingly, the ultraviolet light transmission of the light in an ultraviolet light region to the heat-sensitive layer is reduced and the lightfastness of the heat-sensitive layer is enhanced. On the other hand, since the ultraviolet light absorber does not absorb the visible light, the visible light transmittance in the heat-sensitive layer does not change substantially.
In the general formulae (1) to (4), m represents 1 or 2.
A represents xe2x80x94SO2xe2x80x94R, xe2x80x94COxe2x80x94R, xe2x80x94CO2xe2x80x94R, xe2x80x94CONHxe2x80x94R, xe2x80x94POR1R2, xe2x80x94CH2R3, or SiR4R5R6 in the general formula (1) where m is 1 and in the general formulae (2) to (4). A represents xe2x80x94SO2R7SO2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COCOxe2x80x94, xe2x80x94COR7COxe2x80x94, xe2x80x94SO2xe2x80x94, or SOxe2x80x94 in the general formula (1) where m is 2.
X represents a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, or a halogen atom in the general formulae (1), (3), and (4). X represents an alkylene group, xe2x80x94OR7Oxe2x80x94, or OCOR7CO2xe2x80x94 in the general formula (2).
W represents a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, or a halogen atom in the general formulae (1), (2), and (4). W represents xe2x80x94OR7Oxe2x80x94 or OCOR7CO2xe2x80x94 in the general formula (3).
Y represents a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, or a halogen atom in the general formulae (1), (2), and (3). Y represents xe2x80x94OR7Oxe2x80x94, xe2x80x94OCOR7CO2xe2x80x94, xe2x80x94CH2CH2CO2R7OCOCH2CH2xe2x80x94, xe2x80x94CH2CH2OCOR7CO2CH2CH2xe2x80x94, or CH2CH2CON(R8)R7N(R8)COCH2CH2xe2x80x94 in the general formula (4).
Z represents a hydrogen atom, a halogen atom, an alkyl group, or an alkoxy group.
In the groups listed above, R represents an alkyl group or an aryl group. R1 and R2 each represents an alkoxy group, an aryloxy group, an alkyl group, or an aryl group. R3 represents a phenyl group bearing at least one nitro or methoxy group as a substituent. R4, R5, and R6 each represents an alkyl group or an aryl group. R7 represents an alkylene group or an arylene group. R8 represents a hydrogen atom or an alkyl group.
Among the substituents listed above, the alkyl group may be a straight-chain alkyl group or a branched alkyl group. Further, the alkyl group may have an unsaturated bond. Furthermore, the alkyl group may bear a substituent such as an alkoxy group, an aryloxy group, an alkoxycarbonyl group, an aryloxycarbonyl group, an aryl group, a hydroxyl group, or the like. The aryl group may further bear a substituent such as an alkyl group, an alkoxy group, or a halogen atom.
Among the substituents listed above, the alkylene group may be a straight-chain alkylene group or a branched alkylene group. Further, the alkylene group may contain an unsaturated bond, an oxygen atom, a sulfur atom, or a nitrogen atom. Furthermore, the alkylene group may bear a substituent such as an alkoxy group, a hydroxyl group, an aryloxy group, or an aryl group.
Among the substituents listed above, the arylene group may further bear a substituent such as an alkyl group, an alkoxy group, a halogen atom, or the like.
Among the substituents represented by X, Y, or W, a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms, an aryl group having 6 to 18 carbon atoms, a fluorine atom, a chlorine atom, and a bromine atom are preferable. Among these substituents, a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, a phenyl group, and a chlorine atom are particularly preferable.
Among the substituents represented by Z, a hydrogen atom, a chlorine atom, a fluorine atom, an alkyl group having 1 to 12 carbon atoms, and an alkoxy group having 1 to 12 carbon atoms are preferable. Among these substituents, a hydrogen atom, a chlorine atom, an alkyl group having 1 to 6 carbon atoms, and an alkoxy group having 1 to 6 carbon atoms are particularly preferable.
Among the substituents represented by R, an alkyl group having 1 to 18 carbon atoms and an aryl group having 6 to 18 carbon atoms are preferable. Among these substituents, an alkyl group having 1 to 12 carbon atoms and an aryl group having 6 to 12 carbon atoms are particularly preferable.
Among the substituents represented by R1 or R2, an alkoxy group having 1 to 12 carbon atoms, an aryloxy group having 6 to 12 carbon atoms, an alkyl group having 1 to 12 carbon atoms, and an aryl group having 6 to 12 carbon atoms are preferable.
Among the substituents represented by R3, a 2-nitrophenyl group, a 3,5-dimethoxyphenyl group, and 3,4,5-trimethoxyphenyl group are preferable.
Among the substituents represented by R4, R5, or R6, an alkyl group having 1 to 12 carbon atoms and an aryl group having 6 to 12 carbon atoms are preferable. Among these substituents, an alkyl group having 1 to 8 carbon atoms and a phenyl group are particularly preferable.
In a so-called bis-structure having two benzotriazole rings in one molecule, an alkylene group having 1 to 12 carbon atoms or an arylene group having 6 to 12 carbon atoms is preferable as the substituent represented by R7 and a hydrogen atom or an alkyl group having 1 to 6 carbon atoms is preferable as the substituent represented by R8.
Among the substituents represented by A, xe2x80x94SO2R is particularly preferable.
Specific examples of the above-listed substituent are given below, though it should be understood that the present invention is not restricted to these examples.
Among the substituents represented by X, Y, or W, monovalent substituents include a hydrogen atom, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a t-butyl group, a pentyl group, a hexyl group, an octyl group, a decyl group, a dodecyl group, an allyl group, a 2-butenyl group, a benzyl group, an xcex1-dimethylbenzyl group, a methoxy group, an ethoxy group, a propyloxy group, a butyloxy group, an octyloxy group, a dodecyloxy group, a methoxyethoxy group, a phenoxyethoxy group, a methoxycarbonylethyl group, an ethoxycarbonylethyl group, a propyloxycarbonylethyl group, a butyloxycarbonylethyl group, an octyloxycarbonylethyl group, a phenoxycarbonylethyl group, a phenyl group, a tolyl group, a chlorine atom, a fluorine atom, a bromine atom, and so on. Bivalent substituents include the groups given below.
X: xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94O"Parenopenst"CH2)8xe2x80x94Oxe2x80x94
W: xe2x80x94O"Parenopenst"CH2"Parenclosest"6Oxe2x80x94, 
xe2x80x83xe2x80x94O"Parenopenst"CH2"Parenclosest"2O"Parenopenst"CH2"Parenclosest"2Oxe2x80x94, xe2x80x94O"Parenopenst"CH2"Parenclosest"2S"Parenopenst"CH2"Parenclosest"2Oxe2x80x94
Y: 
xe2x80x83xe2x80x94CH2CH2xe2x80x94CO2xe2x80x94CH2"Parenclosest"4OCOxe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94CO2xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94COxe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94OCO(CH2)8CO2xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH1xe2x80x94CH2xe2x80x94CONH(CH2)6NHCOxe2x80x94CH2xe2x80x94CH2xe2x80x94
Specific examples of the substituents represented by Z include a hydrogen atom, a chlorine atom, a methyl group, an ethyl group, a propyl group, a hexyl group, a methoxy group, an ethoxy group, a propyloxy group, and an octyloxy group.
Specific examples of the monovalent substituents represented by A include a methanesulfonyl group, an ethanesulfonyl group, a butanesulfonyl group, a benzenesulfonyl group, a 4-methylbenzenesulfonyl group, a 2-mesitylenesulfonyl group, a 4-methoxybenzenesulfonyl group, a 4-octyloxybenzenesulfonyl group, a 2,4,6-triisopropylbenzenesulfonyl group, a xcex2-styrenesulfonyl group, a vinylbenzenesulfonyl group, a 4-chlorobenzenesulfonyl group, a 2,5-dichlorobenzenesulfonyl group, a 2,4,5-trichlorobenzenesulfonyl group, a 1-naphthalenesulfonyl group, a 2-naphthalenesulfonyl group, a quinolinesulfonyl group, a thiophenesulfonyl group, an acetyl group, a propionyl group, a butyryl group, a pivaloyl group, a lauroyl group, a stearoyl group, a benzoyl group, a cinnamoyl group, a furoyl group, a nicotinoyl group, a methoxycarbonyl group, an ethoxycarbonyl group, a phenoxycarbonyl group, a hexylaminocarbonyl group, a phenylaminocarbonyl group, a diphenylphosphoryl group, a diethylphosphoryl group, a 2-nitrobenzyl group, a 3,5-dimethoxybenzyl group, a 3,4,5-trimethoxybenzyl group, a trimethylsilyl group, a triethylsilyl group, a t-butyldimethylsilyl group, a diethylisopropylsilyl group, a dimethylphenylsilyl group, a diphenylmethylsilyl group, and a triphenylsilyl group. Specific examples of the bivalent substituents represented by A include the following groups. 
xe2x80x94CO"Parenopenst"CH2"Parenclosest"aCOxe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94
In the case where A is xe2x80x94SiR4R5R6, a photoacid generating agent such as an ammonium salt, a diazonium salt, an iodonium salt, a sulfonium salt, a phosphonium salt, or an onium salt may also be used at the same time in order to enhance photoreactivity. Details of the specific examples of these photoacid generating agents are described in xe2x80x9cOrganic Materials for Imagingxe2x80x9d edited by Research Association of Organoelectronics Materials, 1993.
Specific examples (Specific Examples (1) to (49)) of the compounds represented by the general formulae (1) to (4) are given below, though it should be understood that the present invention is not restricted to these examples.
In a case where m is 2 in the general formula (1) 
When the heat-sensitive recording material is fixed, the compounds represented by the general formulae (1) to (4) do not absorb the fixing light. And, these compounds can enhance light stability of images by absorbing ultraviolet light having longer wavelengths after images are formed.
In the present invention, the compounds represented by the general formulae (1) to (4) can be easily synthesized by conventionally known methods.
In the present invention, in order to inhibit stain formation due to a byproduct that is formed when an ultraviolet light absorber is formed after the compounds represented by the general formulae (1) to (4) react due to the effect of light or heat, preferably a compound represented by the following general formula (5) is also used at the same time. The compound represented by the general formula (5) is a so-called radical trapping agent. If this compound is used, the stain formation can be inhibited. 
In the general formula (5), Ra to Rd each represents a hydrogen atom, an alkyl group, an alkoxy group, an ester group, an aryl group, or a halogen atom. Among these substituents, the alkyl group may be a straight-chain alkyl group or a branched alkyl group. Further, the alkyl group may have an unsaturated bond. Furthermore, the alkyl group may bear a substituent such as an alkoxy group, an aryloxy group, an alkoxycarbonyl group, an aryl group, a hydroxyl group, or the like.
Among the substituents represented by Ra, a hydrogen atom is preferable. Among the substituents represented by Rb, a hydrogen atom is preferable. Among the substituents represented by Rc, a hydrogen atom, an alkyl group, and an aryl group are preferable. Among the substituents represented by Rd. an ester group, an alkyl group, and an aryl group are preferable.
The compound represented by the general formula (5) may be a polar compound or a nonpolar compound. A nonpolar compound is preferable in view of suitability to encapsulation into microcapsules.
Specific examples of the compounds represented by the general formula (5) are given below, though it should be understood that the present invention is not restricted to these examples.
Specific examples of the compounds represented by the general formula (5) include the following structural formula (A), styrene and polymeric oligomers thereof, and the following (A-1 to A-107). Among these, the structural formula (A) and an xcex1-methylstyrene dimer are preferable. The structural formula (A) is more preferable from the standpoint of molecular weight and viscosity. 
In the present invention, the compounds represented by the general formula (5) can be easily synthesized according to conventionally known methods. On the other hand, commercialized products such as xe2x80x9c1A-Axe2x80x9d, xe2x80x9cL-Axe2x80x9d, and xe2x80x9cS-Axe2x80x9d manufactured by Kyoei Sha Yushi Kagaku Co., Ltd. may be used.
In the present invention, a compound represented by any of the general formulae (1) to (4) may be incorporated in any of the layers constituting the heat-sensitive recording material, or alternatively, may be incorporated in an additional layer such as a light and heat controlling layer. Preferably, a compound represented by any of the general formulae (1) to (4) is incorporated in a protective layer, which is disposed above a color-forming layer, from the standpoint of effectively utilizing the function of the protective layer as a light-shielding layer. More preferably, a light transmittance controlling layer is provided on the heat-sensitive layer side opposite to the support side and a compound represented by any of the general formulae (1) to (4) is incorporated in the light transmittance controlling layer. In the case where a protective layer is present, the light transmittance controlling layer is preferable provided between the heat-sensitive layer and the protective layer, in view of, for example, the function as a light-shielding layer of the light transmittance controlling layer.
In the present invention, the coating weight of the compound represented by any of the general formulae (1) to (4) is preferably 0.05 g/m2 or more and more preferably in the range of 0.1 to 1.5 g/m2. If the coating weight is less than 0.1 g/m2, the lightfastness tends to become inferior because the control of the transmission of light, particularly the reduction of the transmittance of the light within an ultraviolet range after irradiation with light whose wavelength is in a range necessary for fixing, is difficult to perform. On the other hand, the coating weight exceeding 1.5 g/m2is disadvantageously excessive.
The compound represented by any of the general formulae (1) to (4) may be used singly or in a combination of two or more.
In the present invention, the content of the compound represented by the general formula (5) is preferably about 0.05 to 20 times, more preferably 0.1 to 5 times, and most preferably 0.3 to 2 times the content of the compound represented by any of the general formulae (1) to (4). If the content of the compound represented by the general formula (5) is less than 0.3 times, the stain inhibiting effect tends to diminish. On the other hand, if the content of the compound represented by the general formula (5) is more than 2 times, the coating weight increases and the stain inhibiting effect per unit content tends to diminish.
The compound represented by the general formula (5) may be used singly or in a combination of two or more.
In the present invention, the compound represented by any of the general formulae (1) to (4) and the hydrogen donor are incorporated into the heat-sensitive recording material according to any of the following methods. That is, (1) a method in which the compound and the hydrogen donor are used as a dispersion of solid particles; (2) a method in which the compound and the hydrogen donor are used as an emulsified dispersion; (3) a method in which the compound and the hydrogen donor are used as a polymer dispersion; (4) a method in which the compound and the hydrogen donor are used as a latex dispersion; and (5) a method in which the compound and the hydrogen donor are used after being encapsulated in microcapsules. Among these methods, (2) a method in which the compound and the hydrogen donor are used as an emulsified dispersion and (5) a method in which the compound and the hydrogen donor are used after being encapsulated in microcapsules are preferable. From the viewpoint of the prevention of the diffusion of the ultraviolet light absorber precursor, (5) a method in which the compound and the hydrogen donor are used after being encapsulated in microcapsules is more preferable.
(2) A method in which the compound and the hydrogen donor are used as an emulsified dispersion is as follows. First, the compound represented by any of the general formulae (1) to (4) and the hydrogen donor are dissolved in an oil. The oil may be a solid, a liquid, or a polymer at normal temperatures. Examples of the oil include low-boiling auxiliary solvents such as acetic ester, methylene chloride, and cyclohexanone and/or phosphoric ester, phthalic ester, acrylic ester, methacrylic ester, other carboxylic acid esters, fatty acid amides, alkylated biphenyl, alkylated terphenyl, alkylated naphthalene, diaryl ethane, chlorinated paraffin, alcohols, phenols, ethers, monoolefins, and epoxies. Specific examples include high-boiling oils such as tricresyl phosphate, trioctyl phosphate, octyldiphenyl phosphate, tricyclohexyl phosphate, dibutyl phthalate, dioctyl phthalate, dilauryl phthalate, dicyclohexyl phthalate, butyl oleate, diethylene glycol benzoate, dioctyl sebacate, dibutyl sebacate, dioctyl adipate, trioctyl trimellitate, acetyltriethyl citrate, octyl maleate, dibutyl maleate, isoamylbiphenyl, chlorinated paraffin, diisopropylnaphthalene, 1,1xe2x80x2-ditolylethane, 2,4-di-t-amylphenol, N,N-dibutyl-2-butoxy-5-t-octylaniline, 2-ethylhexyl hydroxybenzoate, and polyethylene glycol. Among these high-boiling oils, alcohols, phosphoric esters, carboxylic acid esters, alkylated biphenyl, alkylated terphenyl, alkylated naphthalene, and diaryl ethane are preferable. Further, a carbonization inhibitor such as a hindered phenol or a hindered amine may be added to these high-boiling oils. In particular, an oil comprising an unsaturated fatty acid is preferable as the oil and examples thereof include an xcex1-methylstyrene dimer. Examples of the xcex1-methylstyrene dimer include MSD100 (trade name, manufactured by Mitsui Toatsu Chemicals, Inc.).
An oil solution containing the compound represented by any of the general formulae (1) to (4) and the hydrogen donor is added into an aqueous solution of a water-soluble polymer, and the mixture is subjected to emulsifying dispersion by means of a colloid mill, homogenizer, or ultrasonic wave. A water-soluble polymer such as polyvinyl alcohol is used as the water-soluble polymer in the above-mentioned process. In this case, a hydrophobic polymer emulsion or latex or the like can be used together with polyvinyl alcohol. Examples of the water-soluble polymer include polyvinyl alcohol, silanol-modified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, amino-modified polyvinyl alcohol, itaconic acid-modified polyvinyl alcohol, styrene/maleic anhydride copolymers, butadiene/maleic anhydride copolymers, ethylene/maleic anhydride copolymers, isobutylene/maleic anhydride copolymers, polyacrylamide, polystyrenesulfonic acid, polyvinyl pyrrolidone, ethylene/acrylic acid copolymers, and gelatin. Among these water-soluble polymers, carboxy-modified polyvinyl alcohol is particularly preferable. Examples of the hydrophobic polymer emulsion or latex include styrene/butadiene copolymers, carboxy-modified styrene/butadiene copolymers, and acrylonitrile/butadiene copolymers. In this case, a conventionally known surfactant or the like may be added as necessary.
(5) As for the method of encapsulation in microcapsules, a conventionally known method of encapsulation in microcapsules can be employed. The method is as follows. The compound represented by any of the general formulae (1) to (4), the hydrogen donor, and a microcapsule wall precursor are dissolved in an organic solvent which is difficultly soluble or insoluble in water. The resulting solution is added into an aqueous solution of a water-soluble polymer; subjected to emulsifying dispersion by means of a homogenizer or the like; and heated so that a wall film of a polymeric substance constituting a microcapsule wall film is formed in the oil/water interface. Specific examples of the polymeric material constituting a microcapsule wall film include a polyurethane resin, a polyurea resin, a polyamide resin, a polyester resin, a polycarbonate resin, an aminoaldehyde resin, a melamine resin, a polystyrene resin, a styrene/acrylate copolymer resin, a styrene/methacrylate copolymer resin, gelatin, and polyvinyl alcohol. Among these examples, microcapsules having a wall film composed of a polyurethane and/or polyurea resin are particularly preferable.
The microcapsules having a wall film composed of a polyurethane and/or polyurea resin are prepared by the steps of blending a microcapsule wall precursor such as a polyvalent isocyanate into a core material to be encapsulated, subjecting the blend to emulsifying dispersion into an aqueous solution of a water-soluble polymer such as polyvinyl alcohol, and heating the resulting solution so that a polymer forming reaction takes place in the interface of oil droplets.
Some specific examples of the polyvalent isocyanate compound include diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 2,6-tolylene diisocyanate, 2,4-tolylene diisocyanate, naphthalene-1,4-diisocyanate, diphenylmethane-4,4xe2x80x2-diisocyanate, 3,3xe2x80x2-diphenylmethane-4,4xe2x80x2-diisocyanate, xylylene-1,4-diisocyanate, 4,4xe2x80x2-diphenylpropane diisocyanate, trimethylene diisocyanate, hexamethylene diisocyanate, propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, cyclohexylene-1,2-diisocyanate, cyclohexylene-1,4-diisocyanate, 3,3xe2x80x2-dimethoxy-biphenyl diisocyanate, xylylene-1,3-diisocyanate, 4-chloroxylylene-1,3-diisocyanate, 2-methylxylylene-1,3-diisocyanate, cyclohexylene-1,3-diisocyanate, 1,4-bis(isocyanatomethyl)cyclohexane, and 1,3-bis(isocyanatomethyl)cyclohexane; triisocyanates such as 4,4xe2x80x2,4xe2x80x3-triphenylmethane triisocyanate and toluene-2,4,6-triisocyanate; tetraisocyanates such as 4,4xe2x80x2-dimethyldiphenylmethane-2,2xe2x80x2,5,5xe2x80x2-tetraisocyanate; and isocyanate prepolymers such as a hexamethylene diisocyanate/trimethylol propane adduct, a 2,4-tolylene diisocyanate/trimethylol propane adduct, a xylylene diisocyanate/trimethylol propane adduct, and a tolylene diisocyanate/hexanetriol adduct. If necessary, two or more kinds of the polyvalent isocyanate compounds can be used. Among these polyvalent isocyanate compounds, a compound having three or more isocyanate groups in the molecule is particularly preferable.
In the method of encapsulation, as for the organic solvents for dissolving the compound represented by any of the general formulae (1) to (4) and the hydrogen donor, the oils illustrated for use in the emulsifying dispersion can be used. Specific examples of the organic solvent include tricresyl phosphate, trioctyl phosphate, dioctylphthalate, dioctyl sebacate, dioctyl adipate, dibutyl maleate, and O-methylstyrene dimer.
Examples of the water-soluble polymer include polyvinyl alcohol, modified polyvinyl alcohol, methyl cellulose, sodium polystyrenesulfonate, styrene/maleic acid copolymers, gelatin or gelatin derivatives, and polyethylene glycol or polyethylene glycol derivatives.
The particle diameter of the microcapsule is preferably 0.05 to 1.0 xcexcm and more preferably 0.1 to 0.7 xcexcm.
In the method of encapsulation into microcapsules, either the compound represented by any of the general formulae (1) to (4) or the hydrogen donor may be encapsulated or both of the compound represented by any of the general formulae (1) to (4) and the hydrogen donor may be encapsulated. Preferably, both of the compound represented by any of the general formulae (1) to (4) and the hydrogen donor are encapsulated. More preferably, both of the compound represented by any of the general formulae (1) to (4) and the hydrogen donor are enclosed in the same capsule.
In the present invention, in order to lessen the coloration due to browning by light, a compound known as a reducing agent can be used together with the compound represented by any of the general formulae (1) to (4) and the hydrogen donor. Although the reducing agent may be present inside or outside the microcapsule, the reducing agent is preferably present inside the microcapsule. In the case where the reducing agent is present outside the microcapsule, a compound, which penetrates into the microcapsule at the time of heat printing, is used as the reducing agent. Examples of the reducing agent include hydroquinone compounds, hydrazide compounds, hydroxy compounds, phenidone compounds, catechol compounds, resorcinol compounds, hydroxyhydroquinone compounds, pyrrologlycinol compounds, phenol compounds, phenylhydrazide compounds, gallic acid compounds, ascorbic acid compounds, and ethylene glycol compounds. These are described in JP-A Nos.3-191341, 3-25434, 1-252953, 2-302753, 1-129247, 1-227145, 1-243048, 2-262649, etc. Specific examples thereof include N-phenylacetohydrazide, N-phenylbutylylhydrazide, p-t-butylphenol, 2-azidebenzoxazole, and the following compounds. 
In the present invention, the heat-sensitive recording material preferably comprises a support, a photo-fixing type heat-sensitive recording layer, and optional layers such as an interlayer and a protective layer, each provided on the support. It is also possible to provide another layer containing a compound described in JP-A Nos.7-276808, 9-1928, 9-39395, 9-39396, 9-95487, 9-301958, 11-291629, 6-191155, and 12-206644.
In the present invention, preferably the heat-sensitive recording material has a photo-fixing type heat-sensitive recording layer containing a diazonium salt compound whose maximum absorption wavelength is 365xc2x140 nm and a coupler which reacts with the diazonium salt compound to develop a color and a photo-fixing type heat-sensitive recording layer containing a diazonium salt compound whose maximum absorption wavelength is 425xc2x140 nm and a coupler which reacts with the diazonium salt compound to develop a color.
The present invention may also be a heat-sensitive recording material comprising a support having disposed thereon a photo-fixing type heat-sensitive recording layer containing a diazonium salt compound whose maximum absorption wavelength is 425xc2x140 nm and a coupler which reacts with the diazonium salt compound to develop a color and a photo-fixing type heat-sensitive recording layer containing a diazonium salt compound whose maximum absorption wavelength is 365xc2x140 nm and a coupler which reacts with the diazonium salt compound to develop a color, in the order listed.
Further, the present invention may also be a heat-sensitive recording material having a photo-fixing type heat-sensitive recording layer containing a diazonium salt compound whose maximum absorption wavelength is less than 380 nm and a coupler which reacts with the diazonium salt compound to develop a color and a photo-fixing type heat-sensitive recording layer containing a diazonium salt compound whose maximum absorption wavelength is more than 390 nm and a coupler which reacts with the diazonium salt compound to develop a color.
If a plurality of photo-fixing type heat-sensitive recording layers are present, a multicolor heat-sensitive recording material can be obtained by causing the photo-fixing type heat-sensitive recording layers to develop different colors. That is, full-color image recording is possible by selecting the colors to be developed by the photo-fixing type heat-sensitive recording layers such that these colors constitute the three primary colors for subtractive color mixing, i.e., yellow, magenta, and cyan, respectively. In this case, the color developing mechanism of the photo-fixing type heat-sensitive recording layer (the lowermost layer of the photo-fixing type heat-sensitive recording layers) in direct contact with the support is not limited to the combination of an electron-donating dye and an electron-accepting dye. Accordingly, the combination may be, for example, any of the following systems, i.e., a diazo coloration system consisting of a diazonium salt and a coupler which undergoes a color-forming reaction with the diazonium salt; a base-coloration system in which contact with a basic compound develops a color; a color-forming system by chelating; and a color-forming system in which an elimination reaction with a nucleophilic agent causes color formation. It is preferable to provide, on this photo-fixing type heat-sensitive recording layer, two or more photo-fixing type heat-sensitive recording layers each containing a diazonium salt compound having a different maximum absorption wavelength and a coupler which reacts with the diazonium salt compound to develop a color and to superpose on the layers, a layer (light-transmittance controlling layer) containing the compound represented by any of the general formulae (1) to (4) and a protective layer, in the order listed.
The color-forming components to be used in the photo-fixing type heat-sensitive recording layers in the present invention may be conventionally known ones. Particularly preferable are the color-forming components utilizing the reaction between a diazonium salt compound and a coupler, and the color-forming components utilizing the reaction between an electron-donating colorless dye and an electron-accepting compound. Examples of the compounds which are used in the photo-fixing type heat-sensitive recording layer containing a diazonium salt compound and a coupler which reacts with the diazonium salt compound to develop a color when heated, include a diazonium salt compound, a coupler capable of reacting with the diazonium salt to develop a color, and a basic compound which accelerates the reaction between the diazonium salt compound and the coupler. Details of the diazonium salt compound, coupler, and base are described in, for example, JP-B Nos.4-75147, 6-55546, and 6-79867, JP-A Nos.4-201483, 60-49991, 60-242094, 61-5983, 63-87125, 4-59287, 5-185717,7-88356, 7-96671, 8-324129, 9-38389, 5-185736, 5-8544, 59-190866, 62-55190, 60-6493, 60-259492, 63-318546, 4-65291, 5-204089, 8-310133, 8-324129, 9-156229, and 9-175017. Specific examples of the compounds are given below, though it should be understood that the present invention is not restricted to these examples.
(Specific Examples of the Diazonium Salt Compound) 
(Specific Examples of the Coupler) 
(Specific Examples of the Base)
The base may be used singly or in a combination of two or more. Examples of the base include nitrogen-containing compounds such as tertiary amines, piperizines, piperazines, amidines, formamidines, pyridines, guanidines, and morpholines.
Among these compounds, particularly preferred are piperazines such as N,Nxe2x80x2-bis(3-phenoxy-2-hydroxypropyl)piperazine, N,Nxe2x80x2-bis(3-(p-methylphenoxy)-2-hydroxypropyl)piperazine, N,Nxe2x80x2-bis(3-(p-methoxyphenoxy)-2-hydroxypropyl)piperazine, N,Nxe2x80x2-bis(3-phenylthio-2-hydroxypropyl)piperazine, N,Nxe2x80x2-bis(3-(xcex2-naphthoxy)-2-hydroxypropyl)piperazine, N-3-(xcex2-naphthoxy)-2-hydroxypropyl-Nxe2x80x2-methylpiperazine, and 1,4-bis((3-(N-methylpiperazino)-2-hydroxy)propyloxy)benzene; morpholines such as N-(3-(xcex2-naphthoxy)-2-hydroxy)propylmorpholine, 1,4-bis((3-morpholino-2-hydroxy)propyploxy)benzene, and 1,3-bis((3-morpholino-2-hydroxy)propyploxy)benzene; piperidines such as N-(3-phenoxy-2-hydroxypropyl)piperidine and N-dodecylpiperidine; and guanidines such as triphenylguanidines, tricyclohexylguanidines, and dicyclohexylphenylguanidines.
Details of the electron-donating colorless dye, the electron-accepting compound, and others are described in JP-A Nos.6-328860, 7-290826, 7-314904, 8-324116, 3-37727, 9-31345, 9-111136, 9-118073, 11-157221, etc. Specific examples are given below, though it should be understood that the present invention is not restricted to these examples.
(Specific Examples of the Electron-Donating Colorless Dye)

(Specific Examples of the Electron-Accepting Compound)
Examples of the electron-accepting compound include phenol derivatives, salicylic acid derivatives, hydroxybenzoates, etc. In particular, bisphenols and hydroxybenzoates are preferable. Some specific examples thereof include 2,2-bis(p-hydroxyphenyl)propane (i.e., bisphenol A),4,4xe2x80x2-(p-phenylenediisopropylidene)diphenol (i.e., bisphenol P), 2,2-bis(p-hydroxyphenyl)pentane, 2,2-bis(p-hydroxyphenyl)ethane, 2,2-bis(p-hydroxyphenyl)butane, 2,2-bis(4xe2x80x2-hydroxy-3xe2x80x2,5xe2x80x2-dichlorophenyl)propane, 1,1-(p-hydroxyphenyl)cyclohexane, 1,1-(p-hydroxyphenyl)propane, 1,1-(p-hydroxyphenyl)pentane, 1,1-(p-hydroxyphenyl)-2-ethylhexane, 3,5-di(xcex1-methylbenzyl)salicylic acid and polyvalent metal salts thereof, 3,5-di(t-butyl)salicylic acid and polyvalent metal salts thereof, 3-xcex1, xcex1-dimethylbenzylsalicylic acid and polyvalent metal salts thereof, butyl p-hydroxybenzoate, benzyl p-hydroxybenzoate, 2-ethylhexyl p-hydroxybenzoate, p-phenylphenol, and p-cumylphenol.
In the present invention, the methods of using the diazonium salt compound, the coupler which reacts with the diazonium salt compound to develop a color when heated, the basic material, the electron-donating colorless dye, the electron-accepting compound, and the sensitizer are not particularly limited. Examples of these methods include (1) a method in which these compounds are used as a dispersion of solid particles; (2) a method in which these compounds are used as an emulsified dispersion; (3) a method in which these compound are used as a polymer dispersion; (4) a method in which these compounds are used as a latex dispersion; and (5) a method in which these compounds are used after being encapsulated in microcapsules. Among these methods, from the viewpoint of storability, a method in which these compounds are used after being encapsulated in microcapsules is more preferable. In particular, it is preferable to enclose the diazonium salt compound in microcapsules in a color-forming system utilizing the reaction between the diazonium salt compound and the coupler; and it is preferable to enclose the electron-donating colorless dye in microcapsules in a color-forming system utilizing the reaction between the electron-donating colorless dye and the electron-accepting compound.
In the present invention, a plurality of the photo-fixing type heat-sensitive recording layers may be laminated. A multicolor heat-sensitive recording material can be obtained by changing the colors of the constituent photo-fixing type heat-sensitive recording layers. Although the layer construction is not particularly limited, preferable is a multicolor heat-sensitive recording material manufactured by laminating two photo-fixing type heat-sensitive recording layers, with the two heat-sensitive recording layers respectively comprising two different diazonium salt compounds sensitive to respectively different wavelengths and couplers which reacts with the diazonium salt compounds to develop a different color when heated, and a photo-fixing type heat-sensitive recording layer which comprises a combination of an electron-donating colorless dye and an electron-accepting compound. That is, the multicolor heat-sensitive recording material comprise a support having disposed thereon a first photo-fixing type heat-sensitive recording layer containing an electron-donating colorless dye and a electron-accepting compound, a second photo-fixing type heat-sensitive recording layer containing a diazonium salt compound whose maximum absorption wavelength is 365xc2x140 nm and a coupler which reacts with the diazonium salt compound to develop a color when heated, and a third photo-fixing type heat-sensitive recording layer containing a diazonium salt compound whose maximum absorption wavelength is 425xc2x140 nm and a coupler which reacts with the diazonium salt compound to develop a color when heated. In the above-described example, full-color image recording is possible if the colors to be developed in the photo-fixing type heat-sensitive recording layers are selected such that these colors constitute the three primary colors, i.e., yellow, magenta, and cyan, respectively, in the subtractive color mixing.
The method of recording in this multicolor heat-sensitive recording material is as follows. First, the third photo-fixing type heat-sensitive recording layer is heated so that a color is formed by the reaction between the diazonium salt compound and the coupler contained in the layer. Next, the third photo-fixing type heat-sensitive recording layer is irradiated with light having a wavelength of 425xc2x140 nm so that the diazonium salt compound remaining unreacted in the third photo-fixing type heat-sensitive recording layer is decomposed. After that, heat in an amount to cause the second photo-fixing type heat-sensitive recording layer to develop a color is applied to the second photo-fixing type heat-sensitive recording layer so that a color is formed by the reaction between the diazonium salt compound and the coupler contained in the second layer. When the heat is applied, although the third photo-fixing type heat-sensitive recording layer is also heated strongly, the third photo-fixing type heat-sensitive recording layer does not develop any color because the diazonium salt compound is already decomposed to an extent that its color-forming capability is lost. Further, the second photo-fixing type heat-sensitive recording layer is irradiated with light having a wavelength of 365xc2x140 nm so that the diazonium salt compound remaining unreacted in the second photo-fixing type heat-sensitive recording layer is decomposed. be Finally, heat in an amount to cause the first photo-fixing type heat-sensitive recording layer to develop a color is applied to the first photo-fixing type heat-sensitive recording layer. When the heat is applied, although the third and second photo-fixing type heat-sensitive recording layers are also heated strongly, these photo-fixing type heat-sensitive recording layers do not develop any color because the diazonium salt compounds are already decomposed to an extent that their color-forming capability is lost.
In the present invention, in order to further improve the lightfastness, conventionally known antioxidants as described in the following publications can be used. For example, EP 310551A, German Patent Application Laid-Open (OLS) No. 3435443, EP 310552A, JP-A No.3-121449, EP 459416A, JP-A Nos.2-262654, 2-71262, and 63-163351, U.S. Pat. No. 4,814,262, JP-A Nos.54-48535, 5-61166, and 5-119449, U.S. Pat. No. 4,980,275, JP-A Nos.63-113536 and 62-262047, EP 223739A, EP 309402A, EP 309401A, etc.
It is also effective to use additives which are conventionally known in heat-sensitive recording materials and pressure-sensitive recording materials. Some examples of these antioxidants include the compounds described in JP-A Nos.60-125470, 60-125471, 60-125472, 60-287485, 60-287486, 60-287487, 62-146680, 60-287488, 62-282885, 63-89877, 63-88380, 63-088381, 01-239282, 04-291685, 04-291684, 05-188687, 05-188686, 05-110490, 05-1108437, 05-170361, 63-203372, 63-224989, 63-267594, 63-182484, 60-107384, 60-107383, 61-160287, 61-185483, 61-211079, 63-251282, and 63-051174, JP-B Nos. 48-043294 and 48-033212, etc.
A conventionally known binder can be used as the binder in the present invention. Examples of the binder include water-soluble polymers such as polyvinyl alcohol and gelatin, and polymer latices, etc.
A plastic film, paper, plastic resin-laminated paper, synthetic paper, or the like can be used as the support in the present invention.
In the present invention, in the case where heat-sensitive color-forming layers for different colors are laminated, an interlayer can be provided in order to prevent color mixing, etc. In the interlayer, a water-soluble polymeric compound is used. Examples of the water-soluble polymeric compound include polyvinyl alcohol, modified polyvinyl alcohol, methyl cellulose, sodium polystyrenesulfonate, styrene/maleic acid copolymers, and gelatin.
The light transmittance controlling layer and the protective layer in the present invention may contain various pigments, a release agent, and the like besides the water-soluble polymeric compound similar to that for the interlayer.